1. Technical Field
The present invention generally relates to structures of light emitting diodes.
2. Description of Related Art
Light emitting diodes (LEDs) are a type of semiconductor light source that are widely used in electrical products such as scientific instruments, medical instruments, or electronic consumer products, due to their high brightness, long life-span, and wide color gamut. However, the electrical resistivity of a p-type cladding layer of LEDs is comparatively high, resulting in electric current supplied to the LEDs not being uniformly distributed over the p-type cladding layer in a lateral direction and flowing toward an active light emitting layer located below the p-type cladding layer. This is often referred to as current crowding. Current crowding induces light emitted from different areas of the LED having different illuminations which decreases light emitting efficiency of the LED.
Referring to U.S. Pat. No. 5,869,849, an LED which has a decreased current crowding problem is provided. The LED is provided by steps of: i) forming a double hetero-structure of AlGaInP, which includes a p-type AlGaInP cladding layer, an active AlGaInP layer and an n-type AlGaInP cladding layer, on a GaAs substrate; ii) bonding an n-type GaP layer to the double hetero-structure of AlGaInP by a wafer bonding technique, and removing the GaAs substrate from the double hetero-structure of AlGaInP; (iii) sequentially forming a p-type GaAs ohmic contact layer and an indium tin oxide (ITO) current spreading layer on the p-type AlGaInP cladding layer; and iv) respectively forming a first electrical contact on the n-type GaP layer and a second electrical contact on the indium tin oxide current spreading layer.
In the forgoing LED of U.S. Pat. No. 5,869,849, the GaAs substrate, which is an opaque substrate, is replaced by the n-type GaP layer which serves as a transparent window layer of the LED. The light generated by the double hetero-structure of AlGaInP is emitted via the window layer. The n-type GaP layer has good electrical conductivity which decreases current crowding of the LED, and the electric current is dispersed by the indium tin oxide current spreading layer which further decreases current crowding of the LED.
However, in the forgoing LED of U.S. Pat. No. 5,869,849, the GaP layer and the double hetero-structure of AlGaInP are bonded together by directly pressing the GaP layer toward the double hetero-structure of AlGaInP, which causes a bonding force generated between the GaP layer and the double hetero-structure of AlGaInP not adequate enough to fixedly bond the n-type GaP layer with the double hetero-structure of AlGaInP. Therefore, the n-type GaP layer and the double hetero-structure of AlGaInP need to be heated to 650° C. for 30 minutes in the oven so as to fixedly bond the n-type GaP layer with the double hetero-structure of AlGaInP. This will harmfully affect the double hetero-structure of AlGaInP and decrease the yield of the LED.
What is needed, therefore, is a LED which needs not to be heated during manufacturing process and has a high qualifying rate.